In known piezoelectrically actuated fuel injectors, a piezoelectric actuator arrangement is operable to control movement, directly or indirectly, of a valve needle of the injector between injecting and non-injecting states. The valve needle is engageable with a valve needle seating to control fuel delivery through one or more outlet openings of the injector. The piezoelectric actuator arrangement typically comprises a stack of piezoelectric elements, which have an associated capacitance. In a known control technique, varying a voltage applied across the piezoelectric stack controls the energisation level of the stack and therefore the axial length of the stack.
The application of a first voltage across the stack causes the stack to be energised to a first high energisation level in which its length is relatively long. The application of a second voltage across the stack causes the stack to be energised to a second, lower energisation level, and the length of the stack is decreased (i.e. the stack is displaced). By varying the energisation level of the piezoelectric stack, so as to alter stack displacement, movement of the injector valve needle between injecting and non-injecting states can be controlled. The voltages applied to the stack are selected to provide displacement of the stack through an amount that gives the required extent of movement (displacement) of the injector valve needle between its injecting and non-injecting states.
Ideally, every time the voltage across the stack is changed from the first voltage to the second voltage the stack is displaced by the same amount and, similarly, every time the voltage is changed back from the second voltage to the first voltage the stack is displaced back to its original position. In other words, a constant voltage change (from the first voltage to the second voltage) will ideally result in a constant stack displacement (from a first length to a second length). However, in practice this is not the case because many factors affect the stroke per voltage, i.e. the length of stack displacement per unit change in voltage.
As the stack ages, the overall capacitance of the stack reduces resulting in the stack being charged to a lesser energisation level for a given voltage change and hence a reduction in stack displacement. This causes inconsistent fuelling over the lifetime of the fuel injector.
Typically, 90% of the change or drift in the performance of the injector occurs within the first few hours of use. Therefore, it is normal to run the injectors for a number of hours during testing (referred to as the ‘run-in period’), past the majority of the total change. This is not practical during manufacturing. Furthermore, whilst this compensates for the initial drift problem, the performance of the injector continues to drift over the rest of its lifetime due to the stack ageing and so a problem still remains.
To overcome the ageing problem it is known to ‘over-drive’ the stack so that initial displacement of the actuator is greater than the maximum required displacement, but so that this maximum required displacement can still be achieved following several hours of actuator use. It is a disadvantage of this solution, however, that the actuator can be damaged by over-driving so that the service life of the actuator, and hence of the injector, is compromised. It is a further disadvantage that fuel delivery increases when over-driving the stack causing inconsistencies in fuel delivery over time. Furthermore, there is a limit to the maximum voltage that can be applied to the actuator before dielectric breakdown occurs, and also due to limitations in the drive circuits or the acceptable tensile stress in inactive parts of the stack or its external electrodes. This places a limit on the maximum displacement that can be achieved with a new actuator.
In addition, piezoelectric material is temperature sensitive and this has an effect on the overall capacitance, and therefore the stroke per voltage characteristics, of the stack as its temperature varies. This temperature versus stroke/voltage relationship is non-linear and as such it is difficult to counteract this variance.
Furthermore, there can be a significant part-to-part variation in the overall capacitance between different injectors. As a result, it is known to calibrate each part, but this is time and cost intensive.
It is an object of the present invention to alleviate or overcome the problems associated with the changes in the displacement characteristic of a piezoelectric actuator as discussed above, whilst alleviating or avoiding the shortcomings of known techniques used to address this.